Abstract: To avoid the deviation produced in the traditional body-fitted numerical methods and the complex interpolation for sliding mesh on the interface between stator and rotor, a new numerical simulation method was presented for studying on free rotation of rigid body about a fixed axis under fluid force. A mathematical model was established to describe the interaction between immersed rigid body and fluid based on the projection step method. In this paper, the moment source was not pre-calculated, but determined implicitly in such a way that velocity at the immersed boundary interpolated from the corrected velocity field accurately satisfied the no slip boundary conditions, also, the displacement and velocity of rigid body was updated according to the theorem of moment of momentum. The Quick upwind scheme and the second central scheme were applied to solve convection and diffusion terms respectively. The second explicit Adams-Bashforth method was used to the time discretization. Taking the flow over one circular cylinder with Reynolds number 300 as the basic numerical example, the present immersed boundary method(IBM) was validated.